1. Field
The present invention relates generally to data communication, and more specifically to techniques for improving the performance of data delivery to higher layers in conjunction with a hybrid automatic retransmission (HARQ) mechanism in CDMA communication systems.
2. Background
Wireless communication systems are widely deployed to provide various types of services such as voice, packet data, and so on. These systems may be multiple-access systems capable of supporting communication by multiple users, and may be based on code division multiple access (CDMA), time division multiple access (TDMA), frequency division multiple access (FDMA), or some other multiple access techniques. CDMA systems may provide certain advantages over other types of system, including increased system capacity.
To improve the reliability of data transmission, some newer-generation CDMA systems employ a hybrid automatic retransmission (HARQ) mechanism that can retransmit packets that have been incorrectly decoded by the receiver. For example, in W-CDMA Release 5, the HARQ is included in a Medium Access Control (MAC)-hs sublayer that resides on top of the physical layer. On the downlink, an HARQ entity at the transmitter processes data into packets that are assigned sequential transmission sequence numbers (TSNs). These packets may then be transmitted in sequential order based on their TSNs to the receiver.
At the receiver, a corresponding HARQ entity receives the packet transmissions and attempts to decode and recover each transmitted packet. However, due to degradation in the packet transmissions resulting from the radio link, some of the packets may not be decoded correctly (i.e., erased). When this occurs, a negative acknowledgment (NAK) is sent from the receiver back to the transmitter to initiate a retransmission of each erased packet.
The receiver HARQ entity is also tasked with providing the recovered packets (i.e., those decoded correctly) to higher layers. In W-CDMA, the higher layers expect data in the proper order, as determined by the TSNs of the packets. However, with the HARQ mechanism, packets may be recovered out-of-order by the receiver HARQ entity due to retransmissions. As a result, a re-ordering entity is used at the receiver to buffer and re-order the packets as they are recovered by the receiver HARQ. The re-ordering entity would then provide the packets in the proper order, as they become available, to higher layers.
If packets are recovered out of order by the receiver HARQ entity, then the re-ordering entity may “stall” or delay its delivery of the recovered packets to higher layers. In particular, the re-ordering entity would stall the delivery of data to higher layers whenever packets are detected to be missing, until either (1) the missing packets are recovered by the receiver HARQ or (2) the re-ordering entity is confident that the missing packets are lost and will not be recovered by the receiver HARQ. If the second condition is true, then another retransmission mechanism at higher layers may be relied upon to retransmit the lost data.
A challenge arises in determining the proper amount of time to wait by the re-ordering entity before declaring that the missing packets are lost and providing the already recovered packets to higher layers. One goal is to avoid stalling the delivery of data to higher layers, since waiting a long time or indefinitely for lost packets that will not be recovered is undesirable. A short wait time would be better for this goal. A conflicting goal is to minimize erroneous declarations of lost packets, so that unnecessary retransmissions with long delays by higher layers (if supported) or packet loss (if no retransmissions are performed by higher layers) are minimized. A long wait time would provide better assurance that the packets are indeed lost. This problem is commonly referred to in the art as “stall avoidance”.
There is therefore a need in the art for techniques to improve the performance of stall avoidance in a CDMA system.